There are many criteria for classifying peripheral vascular stents, which can be divided into balloo-expandable stent and self-expanding stent based on the mode of stent release: Bare meta stent (BMS), drug coated stent, and covered stent based on the function and therapeutic target of stent: Slotted-tube stent, ring stent, and coiled stent based on the structural design of stent. drug coated stent, Covered stent: according to the structural design of stent, it can be divided into slotted-tube stent, ring stent and coil stent; according to the different mesh of stent, it can be divided into closed-loop stent and open-loop stent. The metal skeleton materials of the stent are generally stainless steel wire, tantalum wire, temperature-controlled nickel-titanium alloy and cobalt-chromium alloy, etc.
I. Ball-expansion type bracket.
The balloon-expanded stent itself is inelastic, and its design is that the stent is pre-installed on the balloon, and the stent is delivered to the vascular lesion through the balloon catheter, and the balloon expands to the proposed diameter and then relies on the retraction force of the vessel wall to adhere to the vessel wall, which does not produce continuous expansion tension on the vessel wall. The greatest advantage of the balloon-expanded stent is its precise positioning during release, which is especially suitable for open lesions, such as the open vertebral artery and the open renal artery, as well as its insignificant post-release shortening and stronger radial support than peripheral self-expanding stents. However, the ball-expandable stent itself lacks elasticity, is prone to collapse and occlusion after compression, and is less flexible, so it is not suitable for extracranial carotid artery, femoral carotid artery and other vulnerable or mobile joint sites; in the peripheral vasculature, it is only suitable for limited short-segment stenotic occlusive lesions (<3 cm) in the non-mobile joint area with straight alignment. Fewer ball-expandable stents are available for peripheral arterial disease, and the classic peripheral ball-expandable stents are represented by the Palmaz (Cordis) and Strecker (Boston) stents.
The Palmaz ball expansion stent and its derivative Genesis series are produced by Cordis and are slotted stainless steel wire mesh tubular stents with a very thin wall thickness of only about 0.15 mm and a closed-loop design. The length is 15 N 50 mm: 5-7 mm in diameter for the renal and vertebral artery openings and 8-14 mm in diameter for the iliofemoral artery. The advantages are strong radial support, close to the vessel wall after expansion, almost no elastic retraction, rapid endothelialization, and less likely to block branch openings. The disadvantage is that the longitudinal flexibility is poor, it is not easy to pass through the twisted vessels, and the overall tendency is straight after release, and it has some resistance to tortuous vessels.
Strecker ball expansion stent is made of O.lmm thin single tantalum wire woven into a tubular metal mesh, the surface has a negatively charged metal oxide layer, which can prevent platelet adhesion, and the X-ray fluoroscopy is clear for accurate positioning. Compared with Palmaz stent, its longitudinal and radial flexibility is good, and it can easily pass through the twisted vessels and adapt to the natural curvature of the vessel wall, and the shortening phenomenon is smaller after expansion, and it is non-ferromagnetic and can be followed up by nuclear magnetic examination. The disadvantage is that the radial support force is smaller than that of Palmaz stent, and there is a certain elastic retraction phenomenon. Therefore, the Palmaz stent with strong support should be used for heavily calcified, occluded and open lesions, and the Strecker stent with good flexibility should be used for obviously distorted lesions.
The Jostent ball-expandable stent (Abbott Vascular) has the advantages of both Palmaz and Strecker stents in that it is easy to position, has strong radial support, adheres closely to the vessel wall after expansion, and is not prone to retraction or displacement: good longitudinal flexibility facilitates smooth delivery into the twisted supra-arch cephalic brachial artery and over the twisted abdominal aortic bifurcation into the contralateral iliac artery. High maneuverability. Another feature is the wide range of stent diameters, which can be expanded from 6 mm to 12 mm.
II. Self-expanding stents.
The release mechanism of the self-expanding 1:l stent is different from that of the ball-expanding stent in that the stent is compressed in the delivery sheath and delivered to the vascular lesion, and the sheath is withdrawn to release the stent, relying on a balanced relationship between the expansion tension of the stent itself and the elastic restriction of the vessel wall so as to adhere to the vessel wall. The advantages of the self-expanding stent are that it is more flexible, facilitates the passage of twisted vessels and calcified lesions, conforms to the natural curvature of the vessel wall, is less susceptible to compression and deformation, and can even be released across mobile joints. The disadvantage is that there is a forward jump and shortening phenomenon during release, which makes it difficult to precisely locate the release.
With the exception of the renal artery and vertebral artery, self-expanding stents are mainly used in the peripheral vasculature, with a wider range of options than ball-expanding stents. The new self-expanding stents are mainly made of nickel-titanium composite, including Symphony stent (Boston Scientific), Luminexx stent (Bard), Zilver stent (Cook), Precise (Cordis), Protégé stent (EV3), and Maris stent (Cordis). Inc.), Maris stent (Invatec Inc.), Sinus stent (Optimed Inc.), etc.
Gianturco-Z-shaped bracket uses stainless steel wire with a diameter of 0.25 to 0.5 mm wound into a cylindrical structure enclosed by Z-bending of various lengths and diameters, which is easy to convey. It is characterized by a large stent mesh, which is less likely to cause obstruction at the opening of vascular branches, strong radial support, and no shortening phenomenon. It is mainly used for venous system lesions, especially for inferior vena cava lesions at the opening of the hepatic vein in Buga syndrome, and is less likely to cause obstruction at the opening of the hepatic vein and the paratentorial hepatic vein. The disadvantage is that it has the phenomenon of forward jump during release, so in order to increase the stability and prevent the stent displacement caused by forward jump, three-section Z-shaped stent should be used routinely. Due to the strong support, it can be used for tough, fibrotic, calcified or highly elastic retracted lesions.
Wallstent stents are made of a mesh tubular structure woven with 0.075 mm diameter stainless steel wire, which has the advantage of good longitudinal flexibility, easy placement in tortuous vessels, and can be used for transarticular placement. The stent can be retrieved and repositioned for re-release when the stent is not released beyond 80% of its full length. The disadvantages are that the stainless steel wire is slim and the fluoroscopic visibility is poor; the radial expansion force is smaller than that of the balloon-expanded stent, and it is not easy to expand certain hard fibrotic or severely calcified lesions, and the post-balloon expansion is needed to ensure that the stent is close to the vessel wall; it is obviously shortened after expansion, and sometimes it is difficult to position; the mesh is smaller and denser than that of similar stents, and it may block the vessel branches.
Memotherm and Smart stents are formed by laser engraving and cutting of nickel-titanium alloy tubes, which have strong radial support, and the stents are shorter and smaller after expansion and have better visibility under fluoroscopy than stainless steel Wallstent. Except for severely calcified stiff lesions and short lesions of limited size (1N 2cm), self-expanding stents with better flexibility should be chosen for iliofemoral carotid arteries and carotid arteries in general. Nickel-titanium alloy self-expanding stents have good overall flexibility and are easier to recover the shape after extrusion than stainless steel self-expanding stents: moreover, the incidence of distant stent fracture may increase due to poor fatigue resistance when stainless steel wire stents are used in carotid arteries.
Bare metal stent.
The metal stent with polished surface and no more coating and laminating material is called bare metal stent, and its introduction can deal with vascular entrapment and acute vascular occlusion very effectively and improve the success rate and safety of angioplasty. Bare stents have proven to be valuable in two ways: as an effective remedy for failure of balloon dilatation angioplasty alone and to reduce postoperative restenosis in the long term. Commonly used metal bare stents include balloon-expandable and self-expandable stents, with the characteristics and types described above. Only the therapeutic principles of stents and their inherent defects are described here.
The limitations of balloon angioplasty are its low immediate success rate in the management of eccentric, calcified, or long-segment stenoses, its high rate of acute intraoperative occlusion, and its high rate of distant restenosis. Early and distant stenosis after PTA is usually due to elastic retraction, endothelial proliferation after endothelial injury, and distant vascular remodeling. Metal bare stents provide effective mechanical support to the vessel wall through their good radial support, thereby eliminating and preventing acute vessel occlusion due to elastic retraction and limited entrapment, providing a larger initial lumen area and smoother intimal surface to allow intra-stent blood flow to hydroxylate and call laminar flow, and limiting restenosis due to negative distal vessel remodeling. Therefore, the introduction of metal bare stents into endoluminal angioplasty after balloons not only effectively reduces the incidence of angioplasty failure and acute vessel occlusion due to elastic retraction and flow-limiting entrapment, ensures surgical safety, and expands the indications for angioplasty, but also helps to maintain long-term patency and reduce long-term restenosis. However, metal bare stents have only mechanical support and lack intrinsic biological activity to inhibit endothelial proliferation, the latter being the main mechanism leading to distant restenosis. In contrast to balloon angioplasty, stents remain in the body’s vascular lumen as foreign bodies for a long time, which can cause excessive intimal proliferation and lead to in-stent thrombosis and long-term restenosis. The lack of biological activity of bare metal stents and the restenosis rate have led to the introduction of new products and design concepts, such as overcoated stents and drug-eluting stents.
IV. Covered stent (covered stent, stent-graft).
Covered stent is composed of a special membrane material covered with polymer on the platform of ordinary bare metal stent, which is an effective combination of the supporting physicochemical properties of bare metal stent and the unique properties of covered material. The stent-type artificial vessel is named specifically for the clad stent used in the aorta. The polymeric membranous materials covered are mainly biodegradable polymers, mainly expandablepolytetrafluoroetnyiene (ePTFE), polyester (pojyethylene terephthalate, PET, commonly known as dacron), polyester ( polyestPE), polyurethane (PU), silk, etc. The diameter of the target vessel has specific requirements for the selection of laminating materials with different properties: for small diameters, resistance to thrombosis is particularly important; for large caliber vessels (≥lOmm) mechanical durability is a relatively prominent issue. Compared to dacron, ePTFE is less thrombogenic and is therefore used for vascular cladding or graft material for diameters ≤lOmm; Dacron has a more pronounced inflammatory and fibroproliferative response than ePTFE and is therefore better tolerated in large-diameter main and iliac arteries. Clad stents or stented prostheses have been widely used for endoluminal repair of dilated arterial disease, such as aortic aneurysms, aortic coarctation and peripheral aneurysms, pseudoaneurysms and arteriovenous fistulas due to vascular injury, and acute rupture perforations due to angioplasty. Since the endothelium can proliferate through the mesh of the bare stent and lead to in-stent restenosis defects, laminated stents are also used in peripheral arterial occlusive disease to restrain the proliferation of endothelium in the stent lumen by the physical barrier effect of the laminated material.
To prevent displacement, barbs are added at both ends of the stent; the length of the stent or stent-type vessel should generally exceed the length of the lesion by more than 2 cm, and the ends of the stent should exceed the ends of the lesion by more than 1 cm, in principle, it is better to be long than short: the diameter of the stent should be larger than the diameter of the vessel at both ends of the lesion by 15% to 20%, so that the stent fits closely with the vessel wall. Compared with the bare metal stent, the outer diameter of the overlapping branch delivery sheath is significantly thicker, usually 8F-12F for peripheral arteries and 16F-24F for aorta.
Stent-type artificial vessels for the aorta are generally self-expanding and releasing, including tubular, bifurcated, and main single iliac types, and are generally supported by means of the cladding material throughout the stent. The stent-type artificial blood vessels mainly include imported Talent (Medtronic) and Zenith (Cook), and N-made Ancura (Shenzhen Xianjian) and Aegis (Shanghai Minimally Invasive). Talent consists of a nickel-titanium alloy self-expanding stent covered with polyester inside and outside, while Zenith consists of Gianturco Z-type stent lined with ePTFE membrane; Both have bare stents proximal to the stent, which can be released across the subclavian and renal artery openings without interfering with branch flow.
The Wallgraft stent (Boston Scientific), based on a Wallstent stent platform covered with polyester, is one of the most commonly used stents, retaining the good radial support of the Wallstent and having good mating properties. It has a good wall attachment performance. The delivery and release technique is basically the same as Wallstent, but the delivery sheath is thicker, about 9-12F, and less flexible. The Hemobahn/Viabahn (W.L. Gore) laminated stent also uses a self-expanding nitinol stent platform with ePTFE lining on the inner surface of the stent and an IOF delivery sheath. Jostent overlaid stent (Jomed) uses a ball-expandable Jostent stent as a platform, covered with ePTFE film on the inner and outer surfaces, and can be delivered via a sheath with an outer diameter of only 9F. 7-8F sheath delivery, with an expanded diameter between 4-12 mm, is often used for deep small-diameter vessels such as coronary arteries, renal arteries, and vertebral arteries.
There are problems with the laminated stent: the outer diameter of the overall delivery sheath is thick and stiff and lacks flexibility, so it is often difficult to deliver or release in twisted vessels; for large arteries, arteriotomy is often required and percutaneous puncture is not possible; local vascular complications increase in general stenting procedures; wrinkling, collapse and breakage of the laminated material cause the weakness of the laminated membrane or the broken area to form stent endothelial proliferation restenosis or internal fistula; when used in peripheral small and medium caliber vessels Early thrombosis is increased, and the cladding material hinders the endothelialization process in the stent lumen leading to late thrombosis: in peripheral arterial occlusive disease stenosis at both ends of the clad stent is still unavoidable.
V. Drug-(J()-eluting stent
Endothelial injury-mediated intimal hyperproliferation is the most important aspect of restenosis. Restenosis consists of three main mechanisms: local injury to the vessel wall triggering excessive cell proliferation and extracellular matrix synthesis (endothelial proliferation): acute elastic retraction immediately after balloon withdrawal; and late vascular remodeling or remodeling leading to an overall reduction in the internal diameter of the vessel. The advent of metal endoprostheses has effectively addressed restenosis due to the latter two mechanisms. The advent of drug-eluting stents has been effective in preventing restenosis due to both early vascular elastic retraction and distant negative remodeling after balloon angioplasty and significantly reducing restenosis due to intimal proliferation. After the drug-eluting stent is placed into the lesion, the anti-smooth muscle cell proliferative drug carried by the polymer carrier encapsulated on the surface of the metal stent is released from the polymer coating in a controlled manner into the lesioned tissue of the local vessel wall to exert biological effects.
The drug-eluting stent consists of three components: the metal stent platform, the polymeric carrier, and the antiproliferative drug. The two main classes of anti-proliferative drugs include rapamycin and paclitaxel. Rapamycin, also known as sirolimus, is a natural macrolide antibiotic that binds to the FK506 protein after diffusion into cells, causing the release and transcriptional arrest of E2F and the reduction of DNA and ribosomal transcriptional protein synthesis in vascular smooth muscle cells, thereby inhibiting smooth muscle cell proliferation: paclitaxel is an anticancer drug, for promoting the binding of microtubule dimers,l preventing microtubule mitosis from proceeding. Other drugs are everolimus and zotamox (Zotorolimus).